Alveolar epithelial injury has been found following exposure to noxious agents as well as during a variety of inflammatory states. This epithelial injury appears to be manifest by damage primarily to type I alveolar cells. This is followed by proliferation of the more resilient type II cells with subsequent regeneration of the epithelium. In addition, the type II cell is the source of the secretory product surfactant, which is important in maintaining alveolar patency and fluid dynamics. Abnormalities in surfactant have been found following acute lung injury, particularly in the adult respiratory distress syndrome (ARDS). Mechanisms involved in producing the injury have not been completely delineated but appear in part to be related to the producton of toxic species released from stimulated neutrophils. The proposed study will therefore investigate possible mechanisms by which toxic mediators released from stimulated neutrophils may induce injury to type II pneumocytes and alter the physiologic properties of secreted surfactant. Rat type II pneumocytes will be isolated in primary culture and used in a cytotoxicity assay system. Isolated cells will be exposed to enzymatically generated reactive oxygen metabolites or neutrophils, stimulated by a variety of mediators. Injury will be assessed by: 1) measuring cellular viability usng a chromium-51 labeling assay; 2) measuring alterations in surfactant synthesis by quantitating the incorporation of 3H-palmitate and 3H-choline into the predominate surfactant components, total and disaturated phosphatidylcholine; 3) measuring the activity of several enzymes involved in surfactant biosynthesis; 4) determination of minimal surface tension and hysteresis profiles of surfactant released from injured type II cells. In addition, the system will be used to identify the specific mediator(s) of toxicity, i.e. either a product of oxygen metabolism and/or a neutrophil-derived protease. This will be assessed by determining if specific antiproteases or scavengers of the products of oxygen metabolism protect the type II cell from neutrophil-induced damage. Elucidation of the pathogenic mechanisms involved in acute inflammatory lung injury may lend insight into methods by which these damaging effects can be circumvented.